Retrofit outcomes matter more than assumptions for UK homes

The gap between a heat pump’s predicted efficiency and its real-world performance can exceed 30%, enough to add £200 a year to a typical 3-bed semi’s heating bill. That discrepancy, as reported by Planning, Building & Construction Today, is the central problem of UK retrofit: the industry has been measuring what is easy to model rather than what actually saves households money.

Why assumptions fail the homeowner

Every EPC certificate in the UK is built on standardised occupancy assumptions, a family of 2.4 people, a thermostat set to 21°C, a fixed number of hours of heating per day. Real homes do not behave that way. A retired couple in a draughty bungalow uses energy differently from a family of four in a modern semi. When retrofit installers specify insulation or a heat pump based on these generic inputs, the promised 40% bill reduction often becomes 15% in practice. Ofgem’s own analysis of the Renewable Heat Incentive found that metered heat pump performance was 22% lower than the deemed calculation on average. That shortfall is not a technical failure, it is a measurement failure.

What a performance-based approach looks like

The shift from assumptions to outcomes requires two concrete changes. First, every retrofit project should include a pre-installation energy audit using smart meter data, not a surveyor’s clipboard. The Energy Saving Trust recommends collecting 12 months of gas and electricity readings before any upgrade. Second, post-installation monitoring for at least one full heating season must be built into the contract. The cost of that monitoring is typically £150–£300 for a basic data logger and analysis, far less than the potential overpayment on an underperforming system. Government schemes like the Boiler Upgrade Scheme currently require only a deemed efficiency calculation. That should change.

The EPC blind spot

England and Wales’ EPC system still uses the Standard Assessment Procedure (SAP), which models fabric performance, not actual consumption. A home can achieve an A rating while its occupants pay £1,800 a year because they work from home and keep the heating on all day. Conversely, a draughty E-rated home with frugal occupants might spend only £1,200. The government has consulted on moving to an ‘Occupancy-Adjusted EPC’ since 2021, but no legislation has followed. Homeowners selling or renting should treat EPC numbers as a guide, not a guarantee. When buying a retrofit property, ask for the vendor’s actual energy bills from the last two winters, not the SAP certificate.

What this misses

Performance-based retrofit is harder to scale. Installers prefer standardised specifications because they are faster and cheaper to deliver. The Green Homes Grant’s failure was partly due to this tension: vouchers were issued based on assumed savings, and when real-world results fell short, trust evaporated. Yet the alternative, continuing to sell homeowners a promise that the building physics cannot keep, is worse. A single household’s disappointment multiplies across a street, a village, a constituency. The retrofit industry will not reach the 600,000 heat pumps a year target by 2028 if every third installation disappoints.

What to do now

Homeowners planning a retrofit should demand a performance contract from their installer: a written agreement that the installed system will achieve a specific kWh/m²/year reduction, verified by before-and-after meter readings. If the installer cannot provide this, find one who can. The Retrofit Academy’s registered practitioners are trained in this approach. For those considering a heat pump under the Boiler Upgrade Scheme, ask your installer for a ‘monitoring plan’ before signing. The grant is £7,500, a monitoring add-on of £200 is a trivial price for knowing whether the system actually works in your home.